1. Field
The present invention relates to a sewing machine having a needle swinging mechanism that swings a needle bar to a left needle location and a right needle location.
2. Related Art
Conventional lock stitch sewing machines capable of zigzag stitch comprise a needlebar, a needlebar vertical movement mechanism and a rotating hook. The needlebar has a lower end to which a sewing needle is attached. A needle has a needle eye through which a needle thread is insertable. The needlebar vertical movement mechanism moves the needlebar upward and downward. The rotating hook seizes a needle thread loop formed near to the needle eye.
A type of sewing machine is provided with a needle swinging mechanism which is capable of swinging a needle right and left between left and right needle locations. In this type of sewing machine, a timing of seizure of a thread loop by a hook beak in the case where a needlebar occupies the left needle location differs from a timing for seizure of a thread loop by the hook beak in the case where the needlebar occupies the right needle location. More specifically, when a location where the hook beak seizes the needle thread loop correspond to an encounter timing, the height of the needle eye relative to the hook beak in synchronization with encounter of the hook beak with the needle thread loop in the case where the needlebar occupies the right needle location is located lower than the needle eye relative to the hook beak in synchronization with encounter of the hook beak with the needle thread loop in the case where the needlebar occupies the left needlebar location. As a result, the hook beak passes through an area where an upper part of the needle thread loop has a smaller width, whereupon there is a possibility that the hook beak cannot reliably hook the needle thread loop thereby to seize the same and accordingly a possibility of stitch skipping.
In view of the foregoing problem, for example, JP-H04-166187 discloses a rotating hook for a sewing machine, in which a non-circular gear provided on an upper shaft is engaged with another non-circular gear provided on an intermediate gear and intermediate and lower shafts are coupled together by a timing belt. As a result, rotational speeds of the upper and lower shafts are set at the same value. The rotational speed of the lower shaft is changed so that the rotational speed of the rotating hook is reduced before the needle thread is reliably seized by the hook beak of the rotating hook. Consequently, the needle thread can reliably be seized by the hook beak of the rotating hook and accordingly, stitches can reliably be formed.
Furthermore, JP-H03-261497 discloses a zigzag sewing machine provided with a needle driving motor, a needle swinging motor and a hook driving motor. The needle driving motor vertically drives a needlebar having a lower end to which a sewing needle is attached. The needle swinging motor swings the needlebar between right and left needle locations. The hook driving motor drives an outer hook serving as a thread loop taker. In this case, the needle driving motor and the hook driving motor are controlled individually. As a result, even in the case where a needle falls onto either right or left needle location, an optimum thread loop which can reliably be seized by a hook beak can be formed.
The non-circular gears of the upper and intermediate shafts are engaged with each other in the sewing machine described in JP-H04-166187. Accordingly, rotating torque becomes non-uniform when rotation of the upper shaft is transferred to the intermediate shaft. As a result, noise due to backlash of the non-circular gears or rattling noise is produced particularly when a sewing speed is increased. Such a noise results in not only worsening of working conditions but also reduction in the durability of the sewing machine.
The needle swinging motor and the hook driving motor need to be controlled individually on the basis of sewing data in the sewing machine described in JP-H03-261497. Accordingly, motor control is complicated. Furthermore, since a plurality of electric motors including the needle swinging motor and the hook driving motor, the weight and size of the sewing machine are increased and the costs of the sewing machine are accordingly increased.